The author's reading of Physics 2.4–6 shows that, for Aristotle, the idea of luck (tuchē) refers to an explanation or a description of action as opposed to a cause of it. More precisely, the idea of luck is invoked when an unexpected outcome appears to have a striking effect on human flourishing in the eyes of some agent or observer. Aristotle's psychological approach to the idea of luck has important implications for his ethical thought. Viewed as an explanation, luck does not necessarily nullify voluntary action. On the author's interpretation of Nicomachean Ethics 3.1, Aristotle argues for an expansive conception of voluntariness that encompasses all actions knowingly initiated–even many actions undertaken in lucky or unlucky circumstances or issuing in lucky or unlucky effects. Aristotle thereby illuminates the seriousness of agency and the task of practical wisdom–to deliberate and to act in accordance with circumstance and to take responsibility for one's own actions without invoking bad luck as an excuse.